To date, the renal handling of amino acids by remnant kidneys remains undefined. We have determined the renal handling of eight amino acids using the isolated perfused rat kidney. 6 normal and 22 partially infarcted rat kidneys (16 stage III [contralateral kidney removed], and 6 stage II [contralateral kidney left intact] were evaluated at perfusate amino acid concentrations approximately ten times normal plasma levels. Despite the reduction in glomerular filtration rate, the urinary excretion of most amino acids in remnant kidneys tended to exceed that of controls. Fractional amino acid excretion by both stage II and III kidneys tended to be or was higher than that of controls. The findings indicate that the fractional reabsorption of amino acids by residual nephrons of remnant kidneys is decreased. Our observations suggest that the changes are unrelated to a functional adaptation to the uremic environment.